The invention concerns a light-emitting diode. In particular, the invention concerns a structured-surface light-emitting diode in which, to improve the uniformity of current supply, an electrical contact layer has a lateral structure by means of which substantially uniform coupling of the electrical current into the light-emitting diode can be achieved.
Light-emitting diodes, such as semiconductor light-emitting diodes (LEDs), are distinguished in particular by the fact that, depending on the system of materials, the internal conversion efficiency of input electrical energy into radiation energy can be very high, i.e., well above 80%. However, the effective decoupling of light from the semiconductor crystal is impaired by the large jump in refractive index between the semiconductor material (typically n=3.5) and the surrounding resin casting material (typically n=1.5). Owing to the resultant small total angle of reflection, of about 26°, at the interface between the semiconductor and the resin casting material, only a fraction of the light generated can be decoupled. With the simple, cubic LED shape typically used in fabrication, a bundle of rays that is not emitted within the roughly 26°-wide decoupling cone remains trapped in the semiconductor crystal, since its angle with respect to the surface normals is not changed even by multiple reflection. As a result, sooner or later the bundle of rays is lost due to absorption, primarily in the area of the contact or the active region or in the substrate. In the case of InGaAIP LEDs, in particular, the absorbing GaAs substrate is a special problem. In conventional LEDs of this kind, the rays emitted from the active region toward the surface of the LED and lying outside the decoupling cone are very likely to be lost in the substrate due to absorption.
The method most commonly used in practice to alleviate this problem is to deposit a thick layer of semiconductor on the top face of the LED. This makes it possible to obtain partial use of the lateral decoupling cones of the emitted luminous radiation.
In U.S. Pat. No. 5,008,718 it is proposed, in an AlGaInP LED, to deposit an electrically conductive GaP layer that is transparent to the emitted luminous radiation on the active, light-emitting layers, primarily to bring about lateral spreading of the current injected through an electrical contact. The advantageous side effect of decreasing total internal reflection and the fact that lateral decoupling of the light beam is made possible by the action of the thick GaP layer have been pointed out elsewhere. In addition, it is proposed to remove the GaAs substrate, which is opaque to the emitted light beam, by etching and to replace at least one transparent layer of the substrate with a suitable material, such as GaP.
The use of one or more thick and transparent layers in a light-emitting diode is also proposed in U.S. Pat. No. 5,233,204. Various configurations for the arrangement and number of these transparent layers are described. These include disposing below the active, light-generating layer a layer realized in a funnel shape and tapering in the direction of the substrate (FIG. 10).
The earliest computer simulations revealed that surface structuring of the topmost thick, transparent semiconductor layer resulted in improved light decoupling values. In particular, surface structuring comprising preferably regularly arranged n-sided prisms, pyramids or frusta of pyramids, cylinders, cones, frusta of cones and the like led to a marked improvement in the decoupling of light. This is because the rays, which initially travel steeply upward, are reflected at the structured surfaces but travel at a lower angle upon each reflection, so that they ultimately are decoupled laterally from the side walls of the structured regions of the top surface.
Such structured-surface light-emitting diodes were initially fabricated by growing the light-generating semiconductor layers on a semiconductor substrate, followed by the upper thick, transparent semiconductor layer, and then depositing a central electrical contact surface on the top surface of the thick semiconductor layer. The top surface of the thick semiconductor layer was then structured by means of etching technology in the areas outside the central contact surface, after which the back of the substrate was thinned and provided with a back contact. This approach proved to be disadvantageous, however, since the thick semiconductor layer, the so-called window, is fragmented by the structuring, causing the current spreading to deteriorate. Thus, there is no adequate distribution of electrical-current in the-regions outside the central contact surface, and the improvement in light decoupling brought about by the structuring is therefore offset by the deficient current spread and the increase in overall luminous flux does not turn out as desired.
It is, accordingly, an object of the present invention to provide a light-emitting diode with high effective decoupling of light. In particular, it is an object of the present invention to simultaneously ensure, in a light-emitting diode, good spatial distribution of the initiated electrical current and good decoupling of the optical light beam.